Volumetric imaging capabilities, such as computed tomography (CT), interventional CT, CT fluoroscopy, 3D vascular imaging, etc., have become increasingly valuable tools over the past decades and years. X-ray CT is a technique which determines the internal make-up of an object by passing X-rays through the object and measuring the attenuation of the X-rays passing through the object. In this technique the object is sub-divided into many voxels, a voxel being a basic volumetric unit for imaging purposes. The number of radiological procedures using X-ray based volumetric imaging techniques has risen accordingly. However, these techniques account for a large fraction of the collective applied dose in radiology and therefore, the applied patient dose has become a critical issue. In order to combat the ever increasing patient dose, efficient dose saving techniques are required.
EP 1 172 069 A1 discloses CT with dose optimization by setting an optimized tube current in real time (automatic exposure control), a tube current modulation (dose minimization), and based thereon a post processing by an adaptive 3D filter (noise reduction). Dose profiles used for acquisition of projection data are calculated based on measured attenuation in the center of a detector. For reconstructed data this leads to a distribution of voxel-noise in such a way that the noise in the center of the reconstructed object is optimal. Even though this technique has found broad acceptance in CT the procedure is suboptimal for several reasons. As a first reason, using only the measured attenuation as a basis for the estimation of the contribution of the individual projection to the total noise in the reconstructed volume is only approximative. Spectral effects originating e.g. from beam hardening and the influence of scattered radiation are neglected. As a second reason, if scatter offset correction techniques are used prior to reconstruction, scattered radiation which represents a significant portion of the measured signal, as well as the impact of the scatter subtraction itself, need to be adequately accounted for in the calculation of the optimum dose profile. As a third reason, if a region other than the central region of the reconstructed volume is the region of interest (e.g., for cardiac CT) the optimum dose profile for the center of the reconstructed image and the region of interest may significantly differ. The dose profile for the center of the reconstructed volume may even decrease the contrast-to-noise ratio in the region-of-interest. It is therefore essential, to do the noise/dose optimization specific for the region of interest.